(60d) Fenton Oxidation Process for the Removal of Microcystin-LR in Nakdong River, Korea

Harmful algal bloom is caused by excessive algal growth that leads producing of harmful toxins. Microcystin-LR is one of the most commonly observed and studied hapatotoxins in surface water. The conventional water treatment processes, including coagulation, flocculation, and sedimentation, are effective for intracellular MC-LR; however, they are not sufficient for removing extracellular MC-LR. Thus, additional chemical processes are required. Advanced oxidation processes (AOPs) are generally more effective for the reduction of MC-LR than conventional oxidation processes. The Fenton oxidation process is an attractive method in AOPs as H₂O₂ is an environmentally friendly oxidant because it decomposes into hydrogen and oxygen and because iron is highly abundant and non-toxic. However, limited studies have been conducted using the Fenton process to remove MC-LR in distilled water. Thus, the aim of this study is the Fenton (Fe(II)/H₂O₂) process was evaluated to investigate the removal of MC-LR in distilled water, and Nak-dong river water from drinking water treatment plant.

The optimal concentrations of Fe(II) and H₂O₂ for the removal of MC-LR were determined as 5 mg/L Fe(II) and 5 mg/L H₂O₂, respectively. The Fenton process is a fast reaction, mostly complete within 5 min (Fe(II), H₂O₂ = 0.2 â?? 10 mg/L); the process had a t_1/2 of less than 5 min at Fe(II) and H₂O₂ concentrations of 5 mg/L and k value of 0.146 min^-1. The degradation intermediates of MC-LR were observed as m/z 1029.5, 1011.5, 835.5, 795.4, and 783.4 using kinetic analysis. The major intermediates were m/z 1029.5 and 1011.5 caused by double hydroxyl addition on the conjugated diene bond of Adda, and single hydroxyl substitution. The second oxidation route tends to remove the Adda chain followed by diene bond cleavage through ·OH attack. The toxicity of microcystin-LR is associated with the diene bond in the Adda chain; thus, cleavage of this bond would be anticipated to alleviate toxicity.

 The effects of the initial MC-LR concentration (2 â?? 200 µg/L) and solution pH (3 - 11) were also examined that a decreasing initial MC-LR concentration and solution pH increased the degradation efficiency of MC-LR. The efficiency of MC-LR degradation was 92.06 ± 0.89% after a 30-min reaction with an initial MC-LR concentration of 2 µg/L. The removal efficiency was highest (77.27%) at pH 3, according to the highest [·OH]_ss (4.2 10^-13 M). In a strong basic solution (pH 9â??11), the removal efficiency was less than 10% because of a decrease in the Fe(II) concentration due to the formation of Fe(OH)₃^-, Fe(OH)⁺,Fe(OH)₂²⁺ and iron hydroxide precipitation

The Nak-dong river water from a drinking water treatment process was also tested. The water sample had the following characteristics for application of Fenton process: DOC = 4.06 mg/L, SUVA value = 2.64 L/mg-m, turbidity = 0.78 NTU, color = 7 Pt-Co unit, Fe (III) = 0.23 mg/L, pH = 7.64, electrical conductivity (EC) = 475 μs/L, geosmin = 23.10 ng/L, and 2-MIB = 25.60 ng/L. The degradation efficiency was 33.13% for a 5-min reaction in raw water from drinking, while the degradation efficiency was lower than that of DI water because of the competing effect (NOM, geosmin, and 2-MIB) and the higher pH. Nevertheless degradation efficiency was lower than that of DI water in every initial MC-LR concentration by the adverse effect of water quality parameters, the degradation efficiency was over 52% with an initial MC-LR concentration of 2 µg/L within 5 min. Therefore, the final concentration of MC-LR was below the WTO drinking water guideline value (1 µg/L) when the initial MC-LR concentration of 2 µg/L in the river water. This study provided knowledge of the Fenton process for use as an alternative cost-effective method to remove MC-LR in drinking water source.